This invention relates to geophysical exploration for petroleum and minerals. More particularly, this invention is directed to geophysical prospecting by means of the seismic technique.
Seismic prospecting involves generating seismic waves at the surface of the earth by means of a seismic source. The seismic, waves travel downward into the earth and are reflected and/or refracted due to differences in acoustic impedance at the interfaces of various subsurface geological formations. Detectors, called seismometers, or geophones, located along the surface of the earth and/or in a borehole produce analog electrical seismic-trace signals in response to detected seismic wave reflections and/or refractions. The analog electrical seismic-trace signals from the seismometers, or geophones, can then be recorded. Alternatively, the analog electrical seismictrace signals from the seismometers, or geophones, can be sampled and digitized prior to being recorded. The seismic-trace data recorded in either manner is subsequently processed and analyzed for determining the nature and structure of the subsurface formations. Specifically, this invention is directed to the acquisition and on-site processing of the seismic-trace data, irrespective of the type of seismic source used for imparting seismic energy to the earth. In particular, this invention is directed to the incorporation of on-site seismic-trace data processing capabilities in the recorder of a cableless seismic digital recording system.
Various forms of cableless seismic digital recording systems are known. The cableless seismic digital recording system is a field system developed for seismic prospecting for digitally recording seismic-trace signals produced by seismometers, or geophones, without the need for multi-conductor cables or alternate means such as multi-channel radio telemetry for transmitting seismic-trace data to a central recording point. In particular, the cableless seismic digital recording system includes recorders placed near the seismometer, or geophone, locations and arranged for producing individual recordings in response to control signals transmitted from a control point over a communications link, preferably a radio communications link.
The forerunner of cableless seismic digital recording systems is disclosed in Montgomery U.S. Pat. No. 3,283,295. Montgomery U.S. Pat. No. 3,283,295 discloses a cableless seismic analog recording system wherein a radio receiver is associated with a recorder located at each seismometer, or geophone, location within a prospect area. Each recorder is activated by control signals from a centrally located radio transmitter. There are, however, two deficiencies in the cableless seismic analog recording system disclosed in Montgomery U.S. Pat. No. 3,283,295, namely, (a) recording of the seismic-trace signals as a frequency-modulated magnetic record is inferior to digital recording, which has unexcelled accuracy, dynamic range, and freedom from noise interference; and (b) there is no provision for selectively activating the remotely deployed recorders for any given recording, so that only certain recorders within a larger array can be employed as in the "roll-along" technique of common-depth-point seismic prospecting. Montgomery U.S. Pat. No. 3,283,295 discloses that all remotely deployed recorders are in operation for every recording, and reconfiguration of the array for each new recording involves physically repositioning various recorders along the profile survey line.
Broding et al. U.S. Pat. No. 3,806,864, hereby incorporated by reference into this specification to form a part thereof, discloses a cableless seismic recording system which overcomes the two noted deficiencies of the cableless seismic analog recording system disclosed in Montgomery U.S. Pat. No. 3,283,295 in that the recording produced is digital in format and, out of a large array of recorders remotely deployed in a prospect area, only those recorders needed for producing a given set of recordings are selectively activated and caused to record the seismic-trace data. The remaining recorders remain essentially quiescent until there is a desire to produce a set of recordings for the prospect areas where they are situated. As disclosed in Broding et al. U.S. Pat. No. 3,806,864, the seismic-trace data is preferably recorded on a magnetic tape cartridge.
Now, many techniques for generating and recording seismic waves are currently in use. Exploding-gas and compressed-air guns placed on the surface of the earth and dynamite are examples of high energy seismic sources which generate a sharp pulse (impulse) of seismic energy. Vibrators, which generate a "chirp" signal of seismic energy, and hammers are examples of low energy surface seismic sources. In the case of vibrators, the recorded seismic wave reflections and/or refractions are cross-correlated with a replica (called the "pilot signal") of the original "chirp" signal in order to produce recordings similar to those which would have been produced with a high energy impulsive seismic source. This process is known as "vibroseis."
Considered in more detail, vibroseis seismic prospecting, commercialized by Continental Oil Company, typically employs a large, vehicle-mounted vibrator as a seismic source. The vehicle is deployed to a prospect area, and the vibrator is positioned in contact with the surface of the earth. Thereafter, the vibrator is activated for imparting vibrations to the earth, thereby causing seismic waves to propagate through the subsurface formations. The seismic wave reflections and/or refractions are detected by seismometers, or geophones, deployed in the prospect area.
Advantageously, the use of a vibrator can be more economical than the use of dynamite. Furthermore, as compared to the use of a high energy impulsive seismic source, such as dynamite, the frequency of the seismic waves generated by a vibrator can be selected by controlling the frequency of the pilot signal to the power source, such as a hydraulic motor, which drives the vibrator. More particularly, the frequency of the pilot signal to the vibrator power source can be varied, that is, "swept," for obtaining seismic-trace data at different frequencies. Consider, for example, Doty et al. U.S. Pat. No. 2,688,124 which discloses how a low energy seismic wave, such as generated by a vibrator, can be used effectively for seismic prospecting if the frequency of the vibrator "chirp" signal which generates the seismic wave is swept according to a known pilot signal and the detected seismic wave reflections and/or refractions are cross-correlated with the pilot signal in order to produce seismic-trace recordings similar to those which would have been produced with a high energy impulsive seismic source. Typically, the pilot signal is a swept frequency sine wave which causes the vibrator power source to drive the vibrator for coupling a swept sine wave "chirp" signal into the earth. A typical swept frequency operation can employ, for example, a 10- to 20-second long sine wave "chirp" signal with a frequency sweep of 14 to 56 Hz. The swept frequency operation yields seismic-trace data which enables the different earth responses to be analyzed, thereby providing a basis on which to define the structure, such as the depth and thickness, of the subsurface formations.
Unfortunately, recorded seismic-trace data always includes some background (ambient) noise in addition to the detected seismic waves reflected and/or refracted from the subsurface formations (referred to as "seismic signal"). Ambient noise is not repeatable with or dependent on the seismic source. The ambient noise appears in many forms, such as atmospheric electromagnetic disturbances, wind, motor vehicle traffic in the vicinity of the prospect area, recorder electrical noise, etc.
When a high energy impulsive seismic source is used, such as dynamite, the level of the detected seismic signal is usually greater than the ambient noise. Use of the cableless seismic digital recording system disclosed in Broding et al. U.S. Pat. No. 3,806,864 is most advantageous in instances when seismic-trace data is generated by a high energy impulsive seismic source. This is because the data storage capacity of commercially available magnetic tape cartridges is adequate for recording the seismic-trace data.
However, when a low energy surface seismic source is used, such as a vibrator used in vibroseis seismic prospecting, the ambient noise can be at a level greater than the seismic signal. For that reason, seismic-trace records are often produced involving the repeated initiation of the low energy surface seismic source at about the same origination point, thereby producing a sequence of seismic-trace data based on seismic wave reflections and/or refractions that have traveled over essentially the same path and therefore have approximately the same travel times. Because the data storage capacity of commercially available magnetic tape cartridges such as used in the cableless seismic digital recording system disclosed in Broding et al. U.S. Pat. No. 3,806,864 is limited, the capacity is not always adequate for recording every repetition individually as well as accommodating the increase in record length required when a low energy surface seismic source is used.
In order to obviate the limitaion of the data storage capacity of commercially available magnetic tape cartridges such as used in the cableless seismic digital recording system disclosed in Broding et al. U.S. Pat. No. 3,806,864, seismic-trace data generated by low energy surface seismic sources can be vertically stacked (summed, or composited) prior to recording in order to economize tape usage. Weinstein et al. U.S. Pat. No. 3,946,357 and Broding U.S. Pat. No. 4,017,833, hereby incorporated by reference into this specification to form a part thereof, disclose hard-wired digital circuitry in the recorder of a cableless seismic digital recording system for vertically stacking seismic-trace data acquired by the recorder. Weinstein et al. U.S. Pat. No. 3,946,357 discloses a recorder including an adder circuit which sums newly acquired seismic-trace data received from a shift register with previously accumulated seismic-trace data temporarily stored in random access memory between consecutive initiations of the seismic source, and the accumulated sum is later recorded on a magnetic tape cartridge. Broding U.S. Pat. No. 4,017,833 discloses a recorder including a plurality of recirculating dynamic shift registers connected in cascade for storing the accumulated sum between consecutive initiations of the seismic source in order to economize power consumption.
The cableless seismic digital recording systems disclosed in Weinstein et al. U.S. Pat. No. 3,946,357 and Broding U.S. Pat. No. 4,017,833 are advantageous in instances when seismic-trace data is generated by high energy impulsive or low energy surface seismic sources. However, the ability to perform operations other than recording, with optional vertical stacking prior to recording, in the recorder of a cableless seismic digital recording system, such as processing the seismic-trace data for further improving the signal-to-noise ratio, is highly desirable. Unfortunately, it is well known that implementation of such features requires substantial modification of and addition to arithmetic and control logic circuits of hardwired digital circuitry such as included in the recorders of the respective cableless seismic digital recording systems disclosed in Weinstein et al. U.S. Pat. No. 3,946,357 and Broding U.S. Pat. No. 4,017,833. Implementation of such features necessitates redesign of the existing hard-wired digital circuitry or a separate component connectable to the existing hard-wired digital circuitry of the recorder. This invention is directed to an alternative approach to facilitate implementation of seismic-trace data processing capabilities in the recorder of a cableless seismic digital recording system.